In a switching regulator as one type of a power amplifier for supplying electric power to a load device, an LC filter is commonly inserted between output terminals of the switching regulator and the load device in order to eliminate noises. When a load connecting line that connects the output terminals of the switching regulator and the load device is long, an equivalent circuitry with the same function as that obtained by inserting the above LC filter is resulted due to floating capacitance and inductance components present in the load connecting line.
FIG. 7 is a circuit diagram representing a circuitry in which an LC circuit is inserted between the output terminals of the switching regulator and the load device. In the figure, a series circuit of switching elements 2, 3 comprising, e.g., MOSFETs are connected across the direct-current power supply 1 with an input voltage vi. By inputting mutually inverted switching pulses from a controller 4 to gates acting as drive terminals of the switching elements 2, 3, the switching elements 2, 3 conduct alternately. A series circuit of a choke coil 5 and a smoothing capacitor 6 is connected with a line between a drain and source of the switching element 3. Both terminals of the smoothing capacitor 6 correspond to output terminals for outputting an output voltage vo. Then, a series circuit of an inductor 7 and capacitor 8 which comprises, e.g., the LC filter and the load connecting line is connected across the smoothing capacitor 6. Both terminals of the capacitor 8 are connected with the load device 9, supplying electric power to the load device 9.
Further, a negative feedback circuit 18 for remotely sensing a load voltage vL is connected with a load connecting line connecting both the terminals of the capacitor 8 and the load device 9. Hereunder is a description of the negative feedback circuit 18. A series circuit of resistors 10, 11 is connected between the load connecting lines. The load voltage vL is divided by the resistors 10, 11 and the voltage thus divided is input to an inverting input terminal of an error amplifier 12. A reference voltage of a reference voltage supply 13 is input to a noninverting input terminal of the error amplifier 12. An output terminal of the error amplifier 12 is connected with a cathode of a photo diode 15. An anode of the photo diode 15 is connected, via the resistor 14, with one terminal, a positive side of the load voltage vL, of the capacitor 8. Further, a capacitor 16 is connected between an output terminal and the inverting input terminal of the error amplifier 12. The photo diode 15 pairs with a phototransistor 17 and thus when the photo diode 15 conducts, a signal caused by the conduction is input to the controller 4 via the phototransistor 17. The feedback circuit 18 functions to feedback comparison information between the load voltage vL and the reference voltage, so that the controller 4 performs the well-known controls such as PWM control and PFM control for a switching pulse input to gates of the switching elements 2, 3.
According to the analogue control described above, there has been a problem that when the LC filter is connected with load of the switching regulator, or the load connecting line is long, sensing of a load side causes output oscillation. FIG. 8 denotes a Bode diagram of the circuit shown in FIG. 7. FIG. 8 shows that a gain at a phase 0 degree is larger than zero and thus oscillation inevitably occurs. As a means for suppressing the oscillation, reducing the whole gain may be considered but degraded response characteristics is unavoidable. If a sensing point is shifted to an output end of an electric power supply, there occurs no oscillation, yet there occurs a voltage drop due to the resistive component of the load connecting line and the inductor, resulting in no desired load voltage vL being obtained.
As a solution to solve this problem, Patent Document 1 discloses a technique in which a target value of an output terminal voltage is calculated in anticipation of a voltage drop in the load connecting line without setting the sensing point at a load end, whereby feedback control is performed in which an influence of the voltage drop in the load connecting line is taken into account.    Patent document 1: Japanese unexamined patent publication No. 9-34561